The present invention relates to a process for producing a cyclic formal by reacting an alkylene glycol with a formaldehyde derivative, and particularly to a process for producing a cyclic formal wherein the amount of impurities produced as by-products at the reaction step can be reduced.
As cyclic formals, there are known, for example, 1,3-dioxolane, 1,4-butanediol formal, diethylene glycol formal, 4-methyl-1,3-dioxolane, 1,3-dioxane, 1,3,5-trioxepane, etc. Hitherto, these are produced by a cyclization reaction of a glycol with an aldehyde, or a cyclization reaction of an alkylene oxide with an aldehyde.
The following processes have been proposed as processes for producing 1,3-dioxolane which is a representative cyclic formal.
West German Patent No.1914209 discloses that 1,3-dioxolane containing 7% of water is obtained in a yield of 96.5% by reacting ethylene glycol with formaldehyde in the presence of an acid catalyst.
Russian Patent No.434737 discloses that 1,3-dioxolane of high purity can be obtained by reacting ethylene glycol with trioxane (hydrous) in the presence of an acid catalyst, subjecting the reaction mixture to extraction with benzene, and additionally subjecting the reaction mixture to washing with a sodium hydroxide solution and rectification.
JP-A-49-62469 discloses that 1,3-dioxolane of high purity is obtained by reacting ethylene glycol with paraformaldehyde in the presence of an acid catalyst, and adding cyclohexane to the reaction distillate liquid, followed by subjecting the reaction mixture to rectification.
However, as a result of an investigation conducted by the inventors, it has been found that in the case of producing a cyclic formal using glycol and formaldehyde as starting materials, if they are reacted using a reactor of the evaporating can type and the vapor after the reaction is continuously discharged, the resulting distillate liquid contains impurities produced at the reaction step and unreacted starting materials (particularly, formaldehyde) in considerably large amounts in addition to the cyclic formal.
The cyclic formal has a property of readily causing azeotropy with water, and, furthermore, if the resulting cyclic formal contains by-product impurities, formaldehyde or the like, purification of the cyclic formal which is a step subsequent to the reaction step becomes further troublesome and complicated. For example, when 1,3-dioxolane is produced using ethylene glycol and trioxane as starting materials, in addition to water as a by-product, there are produced, as impurities, formaldehyde, acetaldehyde, methanol, 2-methyl-1,3-dioxolane, formic acid, 1,4-dioxane, 1,3,5-trioxepane, etc., and all of them incorporate into the distillate liquid. Therefore, in order to obtain 1,3-dioxolane of high purity, separation and removal of these low-boiling point components and high-boiling point components are further needed in addition to separation and removal of water.
The prior art discloses processes for synthesis of a cyclic formal from an alkylene glycol and a formaldehyde derivative, but does not disclose carrying out the reaction while controlling the composition of starting materials used for the reaction at the time when feeding them and at the time of their reaction, thereby inhibiting the production of impurities as by-products. Furthermore, the prior art does not describe the inhibition of the production of impurities by avoiding incorporation of methanol or water into the starting formaldehyde derivative. For example, when 1,3-dioxolane is produced as the cyclic formal, if methanol or formaldehyde is present in the formaldehyde derivative as a starting material, they react with each other to form addition products, and therefore it becomes difficult to separate most of the addition products from 1,3-dioxolane by distillation. In addition, 1,3,5-trioxepane is produced in a large amount during the reaction, and, further, formaldehyde also incorporates into the distillate liquid in a large amount, resulting in reduction of the yield of 1,3-dioxolane.
An object of the present invention is to provide a process for producing a cyclic formal by reacting an alkylene glycol with a formaldehyde derivative, wherein a purification step can be omitted or can be performed easily by reducing the amounts of impurities produced as by-products at the reaction step.
For attaining the above object, the inventors have conducted an investigation on optimum conditions for reacting an alkylene glycol with a formaldehyde derivative in a reaction vessel. As a result, it has been found that the amounts of impurities produced can be reduced when the molar ratio of the alkylene glycol and the formaldehyde derivative as starting materials is within a specific range at the time of feeding and at the time of reaction of the alkylene glycol and the formaldehyde derivative.
Moreover, it has also been found that the vapor produced under the above reaction conditions entrains by-product impurities, formaldehyde or the like, and incorporation of these impurities, formaldehyde or the like into the produced vapor can be considerably inhibited by supplying the produced vapor to a gas-liquid contacting part to allow the vapor to countercurrently contact with a diluent solution.
It has further been found that 1,3-dioxolane of very high purity can be obtained by separating and removing from the produced vapor the high-boiling point components such as unreacted formaldehyde derivative or formaldehyde produced due to decomposition, and further removing water therefrom.
That is, the present invention relates to the following processes.
[1] A process for producing a cyclic formal which comprises feeding an alkylene glycol and a formaldehyde derivative as starting materials to a reaction vessel and reacting the alkylene glycol and the formaldehyde derivative in the presence of a catalyst in the reaction vessel, characterized in that the value of amount of alkylene glycol (mole)/amount of formaldehyde derivative in terms of formaldehyde (mole) is 0.02-0.95 at the time of feeding of the starting materials and is 1.05-50 at the time of reaction of the starting materials.
[2] A process of the above [1], wherein the formaldehyde derivative is trioxane.
[3] A process of the above [1] which further comprises supplying the vapor produced by the reaction of alkylene glycol and formaldehyde derivative to a gas-liquid contacting part, allowing the vapor to countercurrently contact with a diluent liquid, and drawing the diluent liquid after the countercurrent contact from the gas-liquid contacting part without allowing the diluent liquid to flow into the reaction vessel.
[4] A process of the above [3], wherein the gas-liquid contacting part is an absorption tower.
[5] A process of the above [3], wherein the diluent liquid is pure water.
[6] A process of the above [1] which further comprises separating high-boiling point components, unreacted formaldehyde derivative and formaldehyde produced due to decomposition from the vapor produced by the reaction of alkylene glycol and formaldehyde derivative, and removing water from the vapor obtained after the separation.
[7] A process of the above [6] which further comprises condensing the vapor produced by the reaction of alkylene glycol and formaldehyde derivative, separating high-boiling point components, unreacted formaldehyde derivative and formaldehyde produced due to decomposition from the condensate, and removing water from the liquid obtained after the separation.
[8] A process of the above [6], wherein the removal of water from the liquid obtained after the separation is carried out by contacting the liquid with ethylene glycol in a purification tower, and water is added to the liquid before contacting with ethylene glycol.
[9] A process of the above [8], wherein the vapor in the top part of the purification tower has an oxygen concentration of not more than 1000 vol ppm.
[10] A process of the above [3] which further comprises separating high-boiling point components, unreacted formaldehyde derivative and formaldehyde produced due to decomposition from the vapor after being subjected to the countercurrent contacting and removing water from the liquid obtained after the separation.
[11] A process of the above [10] which further comprises condensing the vapor after being subjected to the countercurrent contacting, separating high-boiling point components, unreacted formaldehyde derivative and formaldehyde produced due to decomposition from the condensate, and removing water from the liquid obtained after the separation.
[12] A process of the above [10], wherein the removal of water from the liquid obtained after the separation is carried out by contacting the liquid with ethylene glycol in a purification tower, and water is added to the liquid before contacting with ethylene glycol.
[13] A process of the above [12], wherein the vapor in the top part of the purification tower has an oxygen concentration of not more than 1000 vol ppm.
[14] A process of any one of the above [1]-[13], wherein the cyclic formal is 1,3-dioxolane.